1. Field of the Invention
This invention relates to compositions selected from the quaternary silicate system Al.sub.2 O.sub.3 --CaO--MgO--SiO.sub.2, to the growth of single crystal compounds from this system (hereafter termed "silicates"), and to the utilization of the single crystal silicates, including their use as substrates for epitaxial semiconductor films.
2. Description of the Prior Art
The epitaxial silicon on sapphire (SOS) technology initiated by U.S. Pat. No. 3,393,088, entitled "Epitaxial Deposition of Silicon on Alpha Aluminum", issued to H. M. Manasevit and W. I Simpson, and assigned to the common assignee, has contributed to the rapid growth of silicon semiconductor technology, particularly in the area of microelectronic circuit applications. With the development of this technology, however, there has been imposed increasingly diverse and stringent requirements regarding the physical and chemical properties of the substrate materials. In short, the achievements of silicon on substrate technology and the potential for further application of the technology require the development of additional substrate materials that are compatible with epitaxial films.
Sapphire, of course, has achieved wide acceptance as a substrate material. To date, the focus of attention has been on spinel, as well as sapphire, substrate materials. However, properties such as thermal shock susceptibility and thermal expansion impose limitations on the fabrication parameters and the operating environments of microelectronic circuitry that uses sapphire or spinel substrates. For example, as a result of thermal stability limitations, the surfaces of spinel substrates may decompose during film deposition.
The quaternary silicate system Al.sub.2 O.sub.3 --CaO--MgO--SiO.sub.2 includes compositions that have physical and chemical properties, e.g., thermal expansion coefficients and compatibility with silicon films, that indicate the silicates would be well suited for use as substrates for silicon films and other semiconductor films if the silicates could be grown as single crystals.
The single crystal silicates are not limited to use as substrates. For example, the heat resistant qualities of mullite make it quite suitable for use in heat shields, high temperature particle filters, and so forth.
Various members of the silicate system are used in polycrystalline form, e.g., in the ceramics industry. Several techniques, such as flame fusion, have been used to grow single crystal silicates. However, none of the known techniques has proved satisfactory for the production of large, single crystal boules for device-quality substrates. Growth from the melt, which is used to grow single crystals of materials such as sapphire, has not been successfully applied to the silicates. This lack of success is perhaps due in part to conflicting data in the literature regarding whether the most widely used and best characterized silicate -- mullite -- is incongruently melting and, therefore, incapable of being successfully grown from the melt.
It may be appreciated that it is highly desirable to develop a technique for growing high quality, monocrystalline material from the compositional boundaries of the above-described quaternary silicate system.